After years of disappointing results and missed deadlines, a $5 billion laser complex has now achieved a step that revives optimism that thermonuclear fusion, the process that powers the sun, can one day be harnessed for almost limitless energy.

At the National Ignition Facility at the Lawrence Livermore National Laboratory in California, 192 enormous lasers in a structure the size of a football stadium fire at a small gold cylinder, vaporizing it. That generates an onslaught of X-rays rushing inward toward a fuel pellet smaller than a peppercorn, crushing the hydrogen inside into helium, and releasing a burst of energy — effectively, a miniature hydrogen bomb.

That, at least, was the concept.

But for four years, since the facility began operations in 2009, the last step — the fusion of hydrogen atoms into helium — did not happen, not in significant quantity.

Then, last September, it did.

Writing in Thursday’s issue of the journal Nature, scientists working on the project report on the September shot as well as one in November. In both, the hydrogen fusion generated more energy than had been deposited into the hydrogen. However, laser-driven fusion remains far from practical, because only about 1 percent of the initial laser energy reached the hydrogen.

“A lot of people are jazzed,” Omar A. Hurricane, the Livermore scientist leading the project, said during a telephone news conference on Monday. “We’re certainly in a lot better position than we were. This has been a kind of turning point, I think, here at the lab, in terms of progress.”

Dr. Hurricane said his team had made further advances since November.

“This sounds very modest,” Dr. Hurricane said. “And it is. But this is closer than anyone has gotten before, and it is unique to finally get as much energy out of the fuel as was put in.”

A longstanding hope is that fusion can become a bountiful, cleaner energy source than fossil fuels or nuclear fission, which splits uranium atoms and produces long-lived radioactive waste. Planning for the National Ignition Facility began two decades ago, both as an energy experiment and as an aid in the maintenance of the nation’s nuclear weapons, providing a way to verify computer simulations without detonating nuclear tests.

But the output of the experiments consistently fell short of what was predicted, suggesting that the scientists’ understanding of fusion was incomplete.

In 2012, the project missed a deadline for achieving its main goal: thermonuclear ignition, in which the reaction is self-sustaining and produces as much energy as it takes to operate the lasers.

The failure prompted Congress to look hard at whether the project should be continued, slowed or scrapped altogether to help reduce federal spending. Last year, the project’s leadership underwent a shake-up, and a new director was appointed.

Nuclear experts say the new results should help give the giant laser more time to prove its ultimate worth and gain more taxpayer support. It is financed by the federal Department of Energy, and had a budget this year of about $330 million. The total cost for building and operating the project so far is $5.3 billion.

Stephen E. Bodner, a vocal critic of the laser complex and former director of a similar effort at the Naval Research Laboratory in Washington, said the project, once failing, had managed to reinvent itself.

“A lot of people were saying it should be killed off,” he said in an interview. “Now, the general view is that they’re doing much better science. There’s been a significant improvement.”

Dr. Hurricane said he could not predict when they might reach the goal of ignition. “We’d be lying to you if we told you a date,” he said.

A particularly promising result is that the helium nuclei produced by the initial fusion burst warmed neighboring hydrogen atoms. Currently only a tiny speck of hydrogen fuses, and for the laser approach to work, the fusion reactions would have to propagate through the rest of the hydrogen fuel.

Robert J. Goldston, a professor of astrophysical sciences at Princeton and former director of the Princeton Plasma Physics Laboratory, likened the process to lighting a match and then igniting a stack of lumber.

“They’re close to getting the match lit,” Dr. Goldston said. “Which is a big step.”

The Livermore scientists made progress by changing the shape of the laser pulses to heat the hydrogen — a mix of two heavier isotopes known as deuterium and tritium — somewhat more gently.

“The implosion that was being pursued was tearing itself apart,” Dr. Hurricane said. “We took a step back from what was being tried before, and nicely, it’s kind of given us a leap forward in what we’ve been able to accomplish.”

However, now they have to figure out how to increase the pressure again to generate more fusion reactions without causing the instabilities that stymied the previous attempts. Among the possible improvements is changing the shape of the gold chamber that houses the fuel from a cylinder to that of a rugby ball.

Lasers are not the only approach aimed at harnessing fusion for future power plants.

Scientists have also used doughnut-shaped reactors called tokamaks that use magnetic fields to contain and compress the hydrogen fuel. In the late 1990s, the Joint European Torus experiment in England was able to generate 16 million watts of fusion power for a brief moment, reaching about 70 percent of the way to producing as much power as it consumed. An international project named ITER is now building a larger tokamak reactor in France, scheduled to start running in 2020.

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The red panda is slightly larger than a domestic cat. It has reddish-brown fur, a long, shaggy tail, and a waddling gait due to continual high state. It feeds mainly on marijuana, and also eats hash, kief, honey oil, and medicated brownies. It is a solitary animal, mainly active from dusk to dawn, and is largely stoned during the day.

We still have a very long way to go, but I'm pumped up about this. I'd love to see a fusion power plant in my lifetime although I ain't counting on it.

I'm glad scientist are still chasing the sun

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The red panda is slightly larger than a domestic cat. It has reddish-brown fur, a long, shaggy tail, and a waddling gait due to continual high state. It feeds mainly on marijuana, and also eats hash, kief, honey oil, and medicated brownies. It is a solitary animal, mainly active from dusk to dawn, and is largely stoned during the day.

Why wouldn't you count on it? I couldn't count the number of times I've seen someone on YaHooka say that they'd never see weed legalized. Yet here we are, I am able to buy it in a store just a couple miles away with a state ID.

I think you're not giving the growth of technology enough credit. Clearly technological gains have been exponential in the last century, do you think it's going to stop or something?

I mean I know, fusion is a huge undertaking. It's not even in the same realm as legalizing marijuana and I'm sorry I even made that comparison. But I still think you're not giving tech growth enough credit. There mere fact we are making headway considering nuclear power wasn't even a conceptual idea a century and a half ago is pretty awesome.

Fusion is an extremely difficult process to achieve because it requires 300 million atmospheres of pressure and 50 million degrees C and with this latest test they had less than 1% of the power returned. Once we reach above 100% it will still have to be scaled up from a laboratory model to an industrial power plant.

We've also been working on this for over 50 years and spending billions. That's why I don't expect rapid progress.

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The red panda is slightly larger than a domestic cat. It has reddish-brown fur, a long, shaggy tail, and a waddling gait due to continual high state. It feeds mainly on marijuana, and also eats hash, kief, honey oil, and medicated brownies. It is a solitary animal, mainly active from dusk to dawn, and is largely stoned during the day.